1. Field of the Invention
The present invention relates generally to printing and copying devices, and more particularly relates to the transfer of toner from a rotating photoconductor drum to paper stock being operatively fed through an electrophotographic printer or copier.
2. Description of Related Art
In conventional electrophotographic image reproduction machines, such as printers and copiers, a toner material is depicted onto discharged side surface portions of an otherwise negatively charged rotating photoconductor drum. As these toner-covered drum surface portions are rotated into close adjacency with a side surface of a sheet of paper being fed through the machine, toner from the discharged "image" areas of the drum is electrically drawn onto the paper by a positively charged corotron device positioned adjacent the opposite side of the paper sheet. The sheet is then passed through a heating device which thermally fuses the transferred toner to the paper.
While corotron devices are the conventional and widely accepted means for electrically transferring toner from the photoconductor drum to the paper stock to be toner-imprinted, they are sulot to a variety of well-known problems, limitations and disadvantages. For example, corotrons generate ozone during their operation, thereby creating a potential health hazard for operators of the machines into which they are incorporated. Additionally, corotrons require relatively large amounts of power since their electrical attraction force, which draws toner from the photoconductor drum onto the paper sheet, is created by ionizing the air disposed in the gap between the corotron and the photoconductor drum.
Another limitation associated with the use of corotrons in this application is that they are relatively inefficient in transferring toner from the drum to the paper. For example, of the total quantity of toner deposited on the drum for a given sheet to be printed only about 70-75% is actually transferred to the paper by the typical corotron the toner remaining on the drum after the sheet has been printed is automatically scraped off the drum, dropped into a toner collection housing, and later discarded.
This relatively low toner transfer efficiency of the typical corotron tends to degrade over time, in a relatively rapid fashion, due to unavoidable contamination of the corotron charge wire by dust, moisture and stray toner particles which tend to settle on and adhere to the wire during off periods of the machine. Stray toner and dust settling onto the corotron shield structure over time also tend to undesirably rub off onto the backsides of paper sheet being fed through the machine, leaving unsightly streaks on the paper stock exiting the machine.
As is well known, corotron-based electrophotographic image reproduction machines, such as printers and copiers, are quite susceptible to print quality fluctuations occasioned by changes in ambient air humidity. The usual method of compensating for these print quality fluctuations is to make trial-and error manual adjustments of the "light/dark" setting on the machine until satisfactory print quality is achieved. This manual machine adjustment to compensate for humidity fluctuations is, of course, wasteful of time, energy, paper and toner.
It can readily be seen from the foregoing that it would be desirable to provide improved drum-to-paper toner transfer apparatus and methods in electrophotographic image reproduction machinery which eliminate or at least substantially reduce the above-mentioned problems, limitations and disadvantages heretofore associated with the use of toner transfer corotrons. It is accordingly an effect of the present invention to provide such improved apparatus and methods.